Cavity resonator apparatus



Aug. 24, 1965 v. J. GRANDE CAVITY RESONATOR APPARATUS Filed April 9, 1982 FIG.5

IN VENTOR.

VINCENT J. GRANDE ATTORNEY United States Patent O ferna Filed Apr. 9, 1962, er. No. 135,998 14 Claims. (Cl. 333-83) The present invention relates in general to cavity resonator apparatus and more specifically to an inexpensive, rugged, extremely high Q resonator useful, for example, as a stabilizing or reference resonator for high frequency oscillators, as a highly selective microwave filter, and for other uses.

It is well known that a cavity resonator can simultaneously support resonant fields in a great many modes. The total field configuration within the cavity resonator can comprise a superposition of all possible excited electromagnetic modes which the cavity resonator will support. It is also Well known that some field configurations have ess energy losses associated With them than other field configurations. Energy loss associated with a particular field configuration or mode is due to the IR losses attendant circulating currents in the cavity resonator walls.

One certain family of field configurations, known as the circular electric, or smoke ring family of modes in circular waveguide or more precisely designated as the TEmm,x1 family in circular waveguide wherein m and n may have any integral values, is known to possess less loss associated with its field than any other known Afamily of field conigurations. Within this circular electric family of high Q modes there is one sub family having a higher Q than the remaining modes. This high Q sub family of modes comprises the TEOILn modes wherein n can have any integer value of one or more. If the cavity resonator can be made to oscillate exclusively in TEoyLn modes, the cavity can be made to have an extremely high Q. The problem resides in being able to suppress the unwanted modes of oscillation over a wide range of frequencies for broad tuning range while not lowering the Q of the desired modes.

Heretofore, various mode suppression schemes have been utilized for suppressing unwanted transverse magnetic (TM) and transverse electric (TE) modes. Previously, the unwanted TM family of modes, which are characterized by substantial longitudinal currents in the cavity side walls, were suppressed by the utilization of angular gaps between the resonator end conducting walls and side Walls thereby preventing the fiow of longitudinal currents therebetween. Also, cavity resonator devices have been built wherein the side walls of the cavity resonator are composed of a conductor tightly Wound in a helical configuration and embedded in a lossy material whereby undesired modes of oscillation With longitudinal wall currents may be highly attenuated and prevented from being excited. Other cavity resonators have been built with a slightly curved surface on the end of a tuning plunger which had to be movably supported from an end of the cavity resonator and axially aligned in the cavity to permit tuning.

However, these cavity resonators of the prior art were extremely complex and therefore quite expensive to build. Also, these cavities required substantial mechanical support to prevent microphonics from disturbing the stability which such cavity resonators could provide to other electronic equipment.

Another manner of avoiding many undesired modes is to select the dimensions for a cavity from a region on the right circular cylindrical resonant cavity mode chart wherein most spurious modes are nonexistent. This procedure is often followed in designing TEO'n mode cavities. However, the TMUIn mode resonates simultaneously 3162344 Patented Aug. 24, 1965 With the appropriate TEM,n mode, and it has been necessary to find some means for suppressing the TMLI,n mode, usually by one of the mode suppression schemes set forth above.

According to the present invention, a rugged inexpensive high Q cavity is constructed with at least one hemispherical end wall. Preferably the hemispherical end wall is mounted on one end of a hollow cylinder. The hemispherical end wall is rigid, easy to fabricate and perturbs the TEOJ,n and TMLLn modes diiferently so that these modes no longer resonant sirnultaneously. The resultant cavity is believed to operate on a mode similar to the TEM,n mode for right circular cylindrical cavities and is characterized by a Q which is higher than conventional TEO'U, mode cavities.

As a further provision of this invention the cavity resonator is made tunable by changing the length Vof the cylindrical side wall or by moving or tilting the am's of the cavity end Walls with respect to the aXis of the cylindrical side wall. Also, the cavity is temperature compensated by constructing different parts of the cavity resonator of materials With different thermal coeflicients of expansions whereby radial changes in cavity size are Compensated for by axial changes in cavity size to maintain a Constant frequency over a wide temperature range.

The object of the present invention is to provide a novel rugged, economical high Q cavity resonator being useful as a filter, as a means for stabilizing high frequency oscillators, and the like.

One feature of the present invention is the provision of a hollow cavity resonator provided with a hemispherical end wall which is rugged in use and prevents unwanted modes of oscillation from being excited at the desired frequency.

Another feature of the present invention is the provisionof a novel cavity resonator of the aforementioned feature wherein the hemispherical end wall closes `one end of a hollow cylinder whereby both the desired and undesired resonant modes of oscillation do not exist at the same frequency.

Another feature of the present invention is the provision of a novel cylindrical cavity resonator with a hemispherical end Wall, the hemispherical end wall having the same radius as -the radius of the cylindrical side walls.

Another feature of the'present invention is the provision of a novel cylindrical cavity resonator With a hemispherical end wall and provided With means for changing the length of the cylindrical wall whereby the cavity resonator can be tuned over a relatively wide range.

Another feature of the present invention is the provision of a novel cylindrical cavity resonator with a hemispherical end wall and provided with means for moving 'the axis of an end wall with respect to the axis of the cylindrical side walls for trimming the frequency of the cavity resonator.

Still another 'feature of the present invention is the provision of a cavity resonator of the last aforementioned feature wherein the means for moving the axis of an end wall includes means for eccentrically mounting the hemispherically end wall on the cylindrical cavity side walls whereby the frequency of the cavity resonator can be trimmed by rotation of the hemispherical end wall with respect to the cavity side wall.

Still another feature of the present invention is the provision of a cavity resonator of the second to last aforementioned feature wherein the means for moving the axis of an end wall includes mating portions on the side wall and the end wall with the mating portion contained in a plane other than a plane perpendicular to the axis of the .side walls whereby upon rotation of the end wall with respect to the side wall the axes of the end Wall and the side wall tilt with respect to one another thereby to tune the cavity resonator.

Another feature of the present 'invention is the provision of a novel cavity resonator provided with a hemispherical end wall and temperature compensation means including two walls of materials having different thermal coeicients of expansion whereby radial changes in cavity dimensions are compensated for by axial changes in cavity dimensions thereby to maintain a constant frequency over a wide temperature range.

Other features and advantages of the present invention will become more apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein,

FIG. 1 is a side perspective view showing the novel cavity resonator of the present invention,

PIG. 2 is an exploded view, partially in section, of that structure of FIG. 1 taken along line 2-2 in the direction of the arrows, i

FIG. 2a is an enlarged cross sectional fragmentary view of an alternative embodiment of that portion of the cavity resonator vof FIG. 1 delineated by line 211-211,

FIG. 3 is a side cross sectional view of a further embodiment of the present invention,

FIG. 4 is a side cross 'sectional view of another embodiment of the present invention,

FIG. 5 is a side cross sectional view of still another embodiment of the present invention,

FIG. 6 is a side cross sectional view of still another embodiment of the present invention, and

FIG. 7 is a side elevational view of still another embodiment of the present invention.

Referring now to FIGS. 1 and 2, there is shown a tunable cavity resonator according to the present invention. A cavity resonator 11 is depicted having its side Wall defined by a short hollow metallic cylinder 12, one end of which forms a lip portion for mating With a similar lip portion of a unitary hemispherical end wall portion 13. The side Wall 12 and end wall 13 can be made in two parts, or as is preferred, can be pressed from flat stock as of, for example, 0.050 inch thick Invar, in one operation. The other end of the cylindrical side Wall 12 is closed by a flat circular metallic end wall 14 as of Invar which is provided with input and output coupling irises 15' and 16 for coupling energy into and out of the cavity resonator.

The fiat end wall 14 is a relatively thin plate as of 0.050 inch thick backed up by supporting ribs which construction helps give the cavity resonator a good strength to weight ratio which is especially advantageous for airhorne applications.

The coupling irises are located at a radius of the end wall 14 where the desired TEOJ,n modes have a maximum current density.

In the tunable version of the cavity resonator, shown in FIGS. 1 and 2, the cylindrical side wall 12 is provided with internal threads which screw onto mating threads on the circumferential edge of the end wall 14. A knurled lock nut ring 17 is screwed on over the threaded edge of plate 14- and is used to lock the mating threads of the flat end wall 14 and the side wall 12 once these latter two members have been adjusted to produce the desired length of side wall 12.

Input and output waveguides 18 and 19, respectively, are secured to the outside of the flat end wall 14 surrounding the input and output coupling irises and 16, respectively.

In PIG. 6 the cavity resonator of FIG. 1 is shown as it would be utilized as a stabilizing resonator for stabilizing the oscillating frequency of a high frequency oscillator. Accordingly, as is shown in FIG. 6, a high frequency oscillator 21 such as, for example, a refiex klystron is shown coupled to the cavity resonator via the input waveguide 18'. The output waveguide 19' couples to a load 22.

Referring again to FIG. 1, a phase control screw 23 is provided extending into the input waveguide 18 for adjustably changing the electrical length of the transmission line between the oscillator and the cavity, as desired for maximum stabilization.

Although the coupling irises 15 and 16 are shown as narrow rectangular slots they need not be of this Shape and could be replaced by other and different suitable coupling means such as, for example, coupling loops which are well known in the art.

In operation, the output signal from an oscillator is fed via the input waveguide 18 and the coupling iris 15 to the cavity resonator 11. The cavity resonator, due to its close electromagnetic coupling to the oscillator, serves to stabilize the frequency of the oscillator at the frequency of the excited and coupled to resonant mode of the cavity resonator 11. A portion of the stabilized R.F. signal is coupled from the cavity resonator 11 through the output coupling iris 16 via the output waveguide 19 to the load.

The degree of stabilization which can be produced by a cavity resonator of this type varies directly as the Q of the resonator. Therefore, to achieve high 'stability factors it is necessary that the cavity resonator have an extremely high Q. As was pointed lout previously, if the cavity resonator can be made to operate exclusively on a TEOyLn mode or the smoke ring mode family, the highest Q resonant mode can be provided.

The dimensions of the cavity can be selected from the right circular cylindrical cavity resonator mode chart so as to eliminate many spurious modes other than the undesired TMLLn modes which exist simultaneously with the desired TEOYU, modes in right circular cylindrical cavities. This mode chart can be found in many electronics text books as, for example, on page 912 of Radar Systems and Components, by Bell Telephone Laboratories Staff, D. Van Nostrand Co., New York, 1949. By employment of the hemispherical end wall 13 in the cavity resonator according to the present invention the TMU,n modes are shifted away in frequency from the corresponding TE0,1, so as not to interfere therewith over the tunable range of the cavity.

In a preferred embodiment the cavity shown in FIGS. 1 and 2 is dimensioned to operate on a smoke ring mode similar to the lowest Vorder smoke ring mode in right circular cylindrical cavity resonators designated as the TEMJ, mode. This mode is characterized by a single transverse annular electric field configuration.

In other embodiments the cavity resonator 11 is designed to operate on TEOJ711 modes of higher order than the TEmyl mode by lengthening the cavity. In order to avoid spurious modes which might interfere with higher order TEOJJ, modes and which are characterized by longitudinal side wall currents, an annular gap is provided between the resonator end conducting wall and side walls thereby preventing the flow of longitudinal currents therebetween as set forth in greater detail below with reference to FIG. 2a. Also, for longer cavities operating on higher order TEOJJ, modes certain spurious modes, such as the TEN,n modes, which have a current density minimum at the location of the coupling irises 15 and 16 where the TEM,n modes have a current denstiy maximum, are not excited by energy coupled into the cavity Via the irises and for practical purposes act as though suppressed.

A typical stabilizing cavity resonator built in the manner illustrated in FIGS. 1 and 2 has provided an effective Q of approximately 22,000 and a stability factor of approximately 20 over a frequency range between 8.5 and 9.6 kilomegacycles. Thus the cavity resonator can have a tuning range at least greater than 12% without any appreciable change in the Q. Such a cavity is extremely compact and light weight being approximately 1.94 inches in diameter, approximately one and one-third inches long from the end wall 14 to the top of the hemispherical end wall 13 and weighing less than 4 ounces thereby providing an excellent cavity resonator for airborne application. The Q of such a cavity is higher than the Q of a similar right circular cylindrical cavity resonator operating in the same mode and at the same frequency, the cavity resonator according to present invention having a higher ratio of volume to surface area than a similar right circular cylindrical cavity.

Also, the cavity constructed according to the teachings of the present invention has a mechanical stability approximately ten times that of the comparable right circular cylindrical cavity resonator resulting in excellent frequency stability in a virbrational environment. This is attributable to the hemispherical end wall 13 since the hemisphere is the most rigid type of curved surface which could be supported on the end of the cylindrical side wall. Furthermore, an extrernely rugged cavity is provided when the hemispherical end wall has the same radius as the cylindrical side wall.

Also, the cavity resonator according to the present invention is much more economical to produce since the hemispherical end Wall and side walls can be produced in one operation by pressing them from flat stock.

The flat end wall screw tuning means is extremely rugged and very inexpensive to make. However, instead of using mating threads on the end and side wall the cavity resonator can be tuned by other means which change the effective electrical length of the cavity such as telescoping side walls or the provision of the flat end wall mounted on a movable piston. However, this construction is not as rugged and as inexpensive to build as that described above.

According to a simpler form of the present invention, a fixed tuned cavity resonator can consist of the hemispherical end wall 13 and the proper length of side wall 12 fixedly secured on the flat end wall 14 as, for example, by brazing. Small changes in the resonant frequency of this and the other cavity resonators described herein can be efected by slightly deforming or providing a perturbation as, for example, by dents or screws protruding into the cavity from the hemispherical cavity end wall after the cavity is assembled and on test.

Referring now to FIG. 2a, there is shown an enlarged cross sectional view of the manner in which the cylindrical side wall 12 can be threadably joined to a flat end wall 14' to provide an annular gap 24 therebetween for eifectively suppressing certain undesired transverse magnetic (TM) modes which have longitudinal currents. Although this annular gap is not necessary for successful operation of cavity resonators according to the present invention, it can be helpful for suppressing modes which interfere with the desired mode when the cavity is made larger in order to operate on higher order TEOyLn modes. The end wall 14' is provided with an enlarged diameter portion 25 Wln'ch extends axially outwardly of the cavity and which is provided with threads mating with internal threads on the side wall 12 for adjusting the length of the cavity thereby to vary the resonant frequency thereof. If additional suppression of the undesired TM mode is desired the side walls of the annular gap 24 between the side wall 12 and the end wall 14' are preferably provided with lossy material 26 to suppress the longitudinal currents of undesired modes.

Referring now to FIG. 3 there is shown a further embodiment of the present invention. A cavity resonator 31 includes a first end wall 33, in the Shape of a hollow hemisphere. The open end of this hemisphere is closed by a fiat end wall 34 provided with coupling irises 35 and 36. Although this cavity resonator separates the TEMJ and TMLU modes in frequency, as the other cavity resonators described above and below, the Q of this cavity in FIG. 3 is not as high as the Q of other cavities which include a cylindrical side wall and a hemispherical end wall. It has been found that as the length of the cylindrical side Wall of the cavity resonator illustrated in FIGS. 1 and 2 is reduced to zero the Q of the cavity resonator is also reduced.

Referring now to FIG. 4 there is shown a means for trimming the frequency of the cavity resonator according to the present invention. It has been found that the frequency of cavity resonators according to the present invention can be trimmed by displacing the axis of the hemispherical end wall 43 with respect to the axis of the cylindrical side wall 42. In a convenient manner of accomplishing this the hemispherical end wall 43 is provided with an annular flange 46 and the side wall 42 is provided with a similar annular fiange 47, the annular flanges 46 and 47 being provided with mating annular axially directed shoulder portions 48. The axes of the shoulder portions 48 are displaced eccentrically with respect to the axis of the side wall 42. Thus, upon rotation of the hemispherical end wall 43 the axis of the end wall 43 will be displaced with respect to the axis of the side wall 42.

It has been found that upon displacement of the axis of this hemispherical end wall 43 with respect to the axis of the side wall 42 the resonant frequency of the cavity resonator increases a smallamount. VF or example, it has been found that an axial displacement of 0.050 inch produces a 20 megacycle increase in frequency for a cavity resonator of the size described above with respect to FIGS. 1 and 2 and initially at 8700 megacycles. The tuning has been found to be independent of the direction of displacement. In this manner, the frequency of the cavity resonator can be trimmed slightly 'and then either held by means of a clamping ring 49 or secured in place as, for example, by brazing. In this manner, a single resonant cavity can be produced for a small frequency range and factory tuned at a precise frequency 'in this range by displacing the axis of the hemispherical end wall with .respect to the axis of the cylindrical side wall.

An alternative tuning method and apparatus of the present invention produces a small frequency change, in the resonant frequency of a cavity resonator, as shown in FIG. 5, by tilting the axis of the cavity with respect to the axis of a flat end Wall 54. This is .accomplished by providing a side Wall 52 and flat end wall 54 with slideable mating flange portions 55 and 56, respectively. The abutting sliding surfaces of flanges 55 and 56 are contained in a plane that is not perpendicular to the axis of the cylindrical side wall 52. Thu's, as the isde Wall 52 is rotated with respect to the end wall 54, the axis of the side Wall 52 and the hemispherical end wall 53 will be .tilted With respect to the axis of the end wall 54, as shown in Phantom. It has been found that a 21/2 tilt of the cylindrical side wall 52. Thus, as the side wall of the end wall 54 decreases the frequency megacycles when the cavity frequency is' initially 9350 megacycles. Again the frequency shift is independent of the direction of tilt.

Referring now to FIG. 6, there is shown a cavity resonator according to the present invention which is provided with temperature compensaton for maintaining the resonant frequency of the cavity Constant over a wide temperature range. A shown, the side wall 62 and hemispherical end wall 63 are made of one material with a low thermal coeicient of expansion as, for example, Invar, and at least an |axially outwardly extending portion 65 of the flat end wall 64 is made of a material with a high thermal coefiicient of expansion as, for example, stainless steel. By selecting the proper length for the -axially outwardly extending portion 65 radial changes in cavity size due to changes in temperature can be compensated for by. axial changes in cavity size due to the difierence of thermal coeificients of expansion of the cavity parts in order to maintain the resonant frequency of the cavity resonator constant over a wide temperature range. Also, according to this embodiment of the invention a mode suppressing annular space 66 is easily provided between the side wall 62 and the end wall 64.

The resonant frequency of the typical cavity resonaaaoasaa tor which is constructed according to the present invention and which is not temperature compensated will decrease with an increase in temperature. By way of example, a temperature Compensated cavity resonator of the type shown in PIG. 6 having an approximately 4 inch dameter Invar side wall and an approximately inch long outwardly extending, stainless steel portion 65 has operated at a resonant frequency of 5.9 kilomegacycles and has exhibited only a very slight increase in frequency with a large increase in temperature. By slightly decreasing the length of the outwardly extending portion 65 the resonant frequency of this cavity resonator can be maintained substantially Constant.

As is also illustrated in FIG. 6, the input and output waveguides 18' and 19', respectively, can be of any shape required in the particular application for which the cavity is desired, and the coupling means from Waveguide to cavity can be in the waveguide side walls instead of at the end of the waveguide as shown.

Referring now to FIG. 7, there is shown an alternative embodiment of the present invention wherein both end walls 73 and 74 of the cavity are hemispherical lin shape and the input and output Waveguides 78 and '7% respectively, are provided in the side wall of the cavity. The cavity can be tuned by any one of the several schemes described above. As illustrated in PIG. 7 cylindrical side Wall portions 71 and 72 joined to the hemispherical end walls 73 and 74, .respectively, are joined together at mating edges which are contained in a plane not perpendicular to the axis of the hemispherical end walls 73 and '74 for tuning the cavity resonator by tilting the axes of the end walls 73 and 74.

Furthermore, Whereas the cavity resonator has been illustrated as useful as a stabilization cavity, it can be used as a reference cavity in which case only one coupling iris is necessary for utilization with other microwave equipment.

Also, in the prefer'red embodiments of the invention employing a cylindrical side wall, these side walls need not be perfectly cylindrical but Can be slightly tapered as, for example, from cylindrical without appreciably affecting the Q of the cavity resonator.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustra'- tive and not in a lirniting sense.

What is Claimed is:

1. A high frequency high Q cavity resonator apparatus including, means forming a hollow resonator Chamber, said Chamber means having a hemispherical end wall portion defining one end wal-l of said Chamber and a second end wall por-tion diametrically opposed to said hemispherical end wall -portion Vand defining a second end of said Chamber means, means for exciting and coupling predorninantly to a circular electric mode in said Chamber, said hemispherical end wall portion having an annular lip portion securely mating with a similar annular lip portion on a remaining portion of said Chamber means to form a uni'tary rigid resonant Chamber which is relatively immune to microphonics.

2. The h-igh Q cavity resonator of claim 1 wherein said second end wall is a substantially fiat plate and said ;Coupling means is located in said second end wall.

3. The high Q cavity resonator of claim 1 wherein the said first and second end Wall 'portions are dirnensioned Vof such size `as to support the TEOYU smoke ring mode Without substantially supporting undesired TM modes -resonating simultaneously -at the same frequency, said cou-pling means being -adapted to couple strongly to said TEOJU smoke ring mode.

4. A high frequency high Q resonator apparatus ineluding, means forming a hollow resonator Chamber,

(a) said resonator Chamber means having a cylindrical side wall portion with an vannular lip portion, said Chamber also having a hemispher-ical end wall portion with a similar annular lip portion, said hemisp'herical end Wall por'tion securely mating at its annular lip portion with said annular lip portion of said cylindrical side wall to form a rigid -unitary structure for said Chamber means, and means for exciting and Coupling .to a circular electric mode of resonance in said Chamber.

5. The high Q cavity resonator apparatus of claim 4 wherein the radius of said first hemispherical end wall is substantially equal to the radius of said cylindrical side wall.

6. The high Q cavity resonator apparatus of claim 4 Wherein said second end wall is substantially in the Shape of a hollow hemisphere.

7. The high Q cavity resonator apparatus of claim 4 wherein said second end wall is a fiat plate and said coupling means is located in said fiat plate.

8. The high Q cavity resonator apparatus of claim 4 `including means for changing the length of said cylindrical side wall.

9. The high Q cavity resonator apparatus of claim 4 including means for changing the length of said cylindrical side wall, said means for changing the length of said cylindrical side Wall Comprising mating threads on said side wall and one of said end walls Whereby the end Wall and said side wall can be screwed together and apart thereby changing the effective electrical length of the side Wall to vary the resonant frequency of said cavity resonator apparatus.

10. The high Q cavity resonator apparatus of claim 4 including means for moving the axis of at least one of said end walls with respect to the axis of said cylindrical side wall.

11. The high Q cavity resonator apparatus of claim 4 including means for moving the axis of at least one of said end walls with respect to the axis of the said cylindrical side Wall, said means for moving the axis of one of said end Walls including annular mating portions on said side wall and at least one of said end Walls, the axes of said annular mating portions being spaced from the axis of said side wall Whereby upon rotation of said end wall with respect to said side wall said end wall Will move eccentrically with respect to said side wall thereby to vary the resonant frequency of said cavity resonator apparatus.

12. The high Q cavity resonator apparatus of claim 4 including means for moving the axis of said hemispherical end wall With respect to the axis of said cylindrical side wall, said means for moving the axis of said hemispherical end wall including an annular flange on said hemispherical end wall, an annular flange on said side wall mating with said annular flange on said hemispherical end wall, the axes of said annular flanges being parallel and spaced from the aXis of said side wall whereby upon rotation of said hemispherical end Wall with respect to said side wall said hemispherical end wall will move eccentrically with respect to said side wall thereby to vary the resonant frequency of said cavity resonator apparatus.

13. The high Q cavity resonator apparatus of claim 4 'including means for tilting the axis of at least one of said end walls with respect to the axis of said side wall, said means for tilting the axis of one of said end walls including mating portions on said side wall and at least one of said end walls, said mating portions contained in a plane other than perpendicular to the axis of said cylindrical side wall whereby upon rotation of said end Wall With respect to said side wall the axes of said end wall and said side Wall tilt With respect to one another thereby to vary the resonant frequency of said cavity Aresonator apparatus.

14. The high Q cavity resonator apparatus of claim 4 including means for temperature compensating said cavity resonator, said temperature compensating means nclud- References Cited by the Examiner ing an axially extending portion on said second end wall, UNITED STATES PATENTS said axially extending portion of a material having one thermal coefiicient of expansion, said side wall of a ma- 2'245'669 6/41 Hffllmafm 33,3 83 X terial having a different thermal coeflicient of expansion 5 2500417 3/50 Kmzef 331,* 83 Whereby due to changes in temperature the radial changes 2'600'186 67/52 Bafos 333fl83 2,996,690 8/61 C1a1r 333-83 in the cavity size are compensated by axial changes in cavity size to maintain the frequency of the cavity reso- HERMAN KARL SAALBACH, Primary Examiner. nator Constant over a wide temperature range.

UNITED sTATEs PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 3,202 ,944 August 24, 1965 Vincent J. Grande It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 4, line 45 for HTE0 1h" read TEO, 1,1 column 6, line 49, strike out "52u Thus, as the side wall" and nsert instead axs with respect to the axis Signed and sealed this 6th day of December 1966.

ERNEST w. swmER EDWARD J. BRENNERW Attesting Officer Commissioner of Patents 

1. A HIGH FREQUENCY HIGH Q CAVITY RESONATOR APPARATUS INCLUDING MEANS FORMING A HOLLOW RESONATOR CHAMBER, SAID CHAMBER MEANS HAVING A HEMISPHERICAL END WALL PORTION DEFINING ONE END WALL OF SAID CHAMBER AND A SECOND END WALL PORTION DIAMETRICALLY OPPOSED TO SAID HEMISPHERICAL END WALL PORTION AND DEFINING A SECOND END OF SAID CHAMBER MEANS, MEANS FOR EXCITING AND COUPLING PREDOMINANTLY TO A CIRCULAR ELECTRIC MODE IN SAID CHAMBER, SAID HEMISPHERICAL END WALL PORTION HAVING AN ANNULAR LIP PORTION SECURELY MATING WITH A SIMILAR ANNULAR LIP PORTION ON A REMAINING PORTION OF SAID CHAMBER MEANS TO FORM A UNITARY RIGID RESONANT CHAMBER WHICH IS RELATIVELY IMMUNE TO MICROPHONICS. 